IC with protocol selection memory coupled to serial scan path

ABSTRACT

A digital bus monitor used to observe data on a bus ( 14, 16, 18 ) connecting multiple integrated circuits ( 10, 12 ) comprises a memory buffer ( 30 ), bypass register ( 34 ), test port ( 38 ) and output control circuits ( 42, 46 ) controlled by an event qualifying module (EQM) ( 32 ). In response to a matching condition the EQM ( 32 ) may perform a variety of tests on incoming data while the integrated circuits ( 10, 12 ) continue to operate at speed. A plurality of digital bus monitors ( 20, 22 ) may be cascaded for observation and test of variable width data buses and variable width signature analysis.

This application is a divisional of application Ser. No. 11/177,663,filed Jul. 7, 2005, now U.S. Pat. No. 7,013,416 which was a divisionalof application Ser. No. 10/690,325, filed Oct. 21, 2003, now U.S. Pat.No. 6,990,620 which was a divisional of application Ser. No. 10/649,274,filed Aug. 27, 2003, now U.S. Pat. No. 6,959,408, issued Oct. 25, 2005;which was a divisional of application Ser. No. 09/597,472, filed Jun.20, 2000, now abandoned; which was a divisional of application Ser. No.09/265,028, filed Mar. 9, 1999, now U.S. Pat. No. 6,131,171, issued Oct.10, 2000; which was a divisional of application Ser. No. 08/929,389,filed Sep. 15, 1997, now U.S. Pat. No. 5,905,738, issued May 18, 1999;which was a continuation of application Ser. No. 08/350,933, filed Dec.7, 1994, now abandoned; which was a continuation of application Ser. No.07/892,392, filed May 28, 1992, now abandoned; which was a continuationof application Ser. No. 07/708,099, filed May 24, 1991, now abandoned;which was a continuation of application Ser. No. 07/374,896, filed Jun.30, 1989, now abandoned.

BACKGROUND OF THE INVENTION

Traditionally, expensive test equipment has been required to dynamicallymonitor the functional interactions of integrated circuits on a circuitboard design. During test, the integrated circuits are made to operatetogether functionally while an external tester observes the transactionsoccurring between the integrated circuits. Other techniques such asboundary scan provide off-line or static testing of wiring interconnectsbetween integrated circuits, but are not effective in detecting at-speedfunctional problems that can occur in the bussing paths betweenintegrated circuits.

The ability to dynamically (i.e., during normal operation of the circuitboard) observe the data passing between integrated circuits in real-timeallows monitoring of the functional interactions between multipleintegrated circuits on a substrate, such as a circuit board. Such a testcan reveal timing sensitive and/or intermittent failures that wouldotherwise not be detectable without the use of expensive testers andmechanical probing fixtures. The ability to dynamically observe systemdata buses in real-time facilitates system integration, environmentalchamber testing, remote diagnostic testing, and built-in self testing.

Heretofore, the ability to dynamically observe digital signals on busesformed of leads on the substrate or circuit board between integratedcircuits has been unavailable. Current test methods used to monitor orobserve digital signals on signal paths or leads between integratedcircuits on substrate or circuit board designs require the use ofexpensive external tester equipment and signal node probing mechanisms.One problem with the current test approach is that state-of-the-artcircuit board designs are so densely populated with integrated circuitsthat physical probing of the signal paths is very difficult, if notimpossible. Another problem is that the circuit board tester isdependent upon the availability of external testers and probingfixtures. Transportation and upkeep of the required test equipment in afield environment to support a system can be a very expensiveproposition.

Therefore, a need has arisen to provide a digital bus monitoring devicewhich can be used to dynamically observe data or address signals on abus leads connecting multiple integrated circuits.

SUMMARY OF THE INVENTION

In accordance with the present invention, a digital bus monitor isprovided which substantially eliminates or prevents the disadvantagesand problems associated with prior digital bus testing devices.

The digital bus monitor of the present invention may be used to observedata on a bus connecting multiple integrated circuits while theintegrated circuits are in a functioning mode. Input circuitry isprovided for receiving incoming data. Test circuitry is connected to theinput circuitry for analyzing and storing data in response to detectionof a known or certain condition. The known or certain condition may bedetected by comparing data from the logic circuitry to an expected dataword stored in a register or memory. Some bits of the expected data wordmay be masked using a masking data word, such that the masked bits arenot involved in the matching operation. Multiple digital bus monitordevices may be cascaded together to allow for observation and test ofvariable width data buses.

The digital bus monitor of the present invention provides severaltechnical advantages over the prior art. The digital bus monitors of thepresent invention may be imbedded in the circuit board or substratedesign and can be activated throughout the life cycle of the substrateor circuit board, from production testing to field service andmaintenance. Another advantage is that the digital bus monitors do notimpede the performance of the substrate's or board's circuitry. Sincethe input signals to be monitored do not have to be routed through thedigital bus monitor, but are only input to the digital bus monitor, nosignificant functional performance penalty is paid while using thesedevices.

In the second embodiment of the present invention, a second known orcertain condition may be detected, at which time the storage andanalysis will cease. The storage and analysis may be resumed afterdetection of a third known or certain condition and stop after thedetection of a fourth known or certain condition.

The digital bus monitor of the present invention provides the advantageof analyzing signals or data on data or address buses coupling multipleintegrated circuits while the integrated circuits are operatingat-speed. The at-speed testing of the integrated circuits detect errorsthat might not otherwise be found.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of an application of the presentinvention illustrating two digital bus monitor devices coupled to thedata and address bus of two integrated circuits;

FIG. 2 illustrates a block diagram of the digital bus monitor integratedcircuit;

FIG. 3 illustrates a block diagram of the test port used in the presentinvention;

FIG. 3 a illustrates a timing diagram for the command registerload/shift scan operation;

FIG. 3 b illustrates a timing diagram for the data register load/shiftscan operation;

FIG. 4 illustrates a block diagram of the command register used in thepresent invention;

FIG. 5 illustrates a block diagram of the bypass register used in thepresent invention;

FIG. 6 illustrates a block diagram of the test cell control registerused in the present invention;

FIG. 7 illustrates a block diagram of the event qualification moduleused in the present invention;

FIG. 7 a illustrates a timing diagram of a single test monitor operationin response to a condition input;

FIG. 7 b illustrates a timing diagram of a test monitor operation whilethe condition input is present;

FIG. 7 c illustrates a timing diagram of a test monitor operationbetween start and stop condition inputs;

FIG. 7 d illustrates a test monitor operation started with a firstcondition input, paused with a second condition input, resumed with athird condition input, and stopped with a fourth condition input;

FIG. 8 illustrates a block diagram of a memory buffer used in thepresent invention;

FIG. 9 illustrates a block diagram of the test cell register used in thepresent invention;

FIG. 9 a illustrates a block diagram of a test cell used in the presentinvention;

FIG. 9 b illustrates a block diagram of the exclusive or gate networkused in the test cell register of the present invention;

FIG. 9 c illustrates a block diagram of signature analysis performed onall ODI inputs;

FIG. 9 d illustrates a block diagram of signature analysis performed onan isolated ODI input; and

FIG. 10 illustrates a block diagram of the digital bus monitor feedbackcontrol for cascaded and non-cascaded PSA used in the present invention.

DETAILED DESCRIPTION OF INVENTION

The preferred embodiment of the present invention is best understood byreferring to FIGS. 1-10 of the drawings, like numerals being used forlike and corresponding parts of the various drawings.

Digital Bus Monitor Application

FIG. 1 illustrates a block diagram of an exemplary circuit using thedigital bus monitor (DBM) of the present invention. As illustrated inFIG. 1, two integrated circuits, IC1 10 and IC2 12, are connected bythree buses: An address bus 14, a data bus 16, and a control bus 18.Each bus is formed of plural leads affixed to a substrate, such as acircuit board. A first data bus monitor DBM1 20 is connected to thecontrol bus 18 by its clock or CK input and to the data bus 16 via itsODI (observability data input) input. The second data bus monitor DBM222 is connected to the control bus 18 via its clock CK input and to theaddress bus 14 via its ODI input. DBM1 20 and DBM2 22 are interconnectedtogether via a serial scan path connection comprising a test data input(TDI) and a test data output (TDO). The TDO of DBM1 20 is connected tothe TDI of DBM2 22. Control for operation of the DBMs 20 and 22 for scanand off-line test operations is input via the test clock (TCK) and testmode select (TMS) inputs. The TDI, TDO, TCK and TMS scan path signalsare compatible with a proposed IEEE standard test bus for integratedcircuits. An event qualification output (EQO) is output from each of theDBMs 20 and 22 into an AND gate 24. The output of the AND gate 24 isinput to the event qualification inputs (EQI) of each DBM 20 and 22.TMS/TCK and TDI may be supplied by an external test bus controller 25.TMS/TCK and TDI may optionally be input to an IC with internal testcircuitry. TDO and EQI are received by the test bus controller from DBM222 and AND gate 24, respectively.

In operation, the DBMs 20 and 22 are used to observe and test thedigital signals carried on the buses 14-18 connected between the twointegrated circuits 10 and 12. DBM devices operate in two modes:off-line test mode and on-line test mode. In the off-line mode, boardcircuitry is placed in a test mode and control for signal monitoring isinput to the DBMs 20 and 22 from the external test bus interface. Theexternal test bus interface includes four signals: TCK, TMS, TDI, andTDO. TCK and TMS are the test clock and test mode select signals,respectively, from the external test controller. TDI and TDO are theserial test data input and output signals used to connect DBMs and otherdevices conforming to the IEEE interface specification. Using theon-line mode, the board circuitry is functioning normally, and controlfor monitoring comes from the DBMs internal event qualification module(EQM) which is described in detail in connection with FIG. 7.

In the exemplary circuit of FIG. 1, IC1 10 outputs address and controlsignal information to IC2 12 to allow data signals to pass between thetwo integrated circuits. First and second DBMs 20 and 22 are included inthe circuit of FIG. 1 to provide for monitoring the signals on the dataand address paths or leads between IC1 10 and IC2 12. The address anddata bus signals to be monitored are input to the DBMs via ODI. If theDBMs were not included in the circuit example of FIG. 1, external probeswould have to be connected to these data paths or leads to achieve thelevel of signal observability provided by the DBMs.

When the circuits are placed in an off-line test mode, IC1 10 and IC212, can be controlled so that the address and data bus paths or leads 14and 16 can be monitored by the first and second DBMs 20 and 22. Duringthe test, IC1 10 can be made to output data on its address and databuses or leads 14 and 16. The data and address output from IC1 10 can becaptured into both DBMs 20 and 22 via ODI 14, 16 inputs. After the datahas been captured, it can be shifted out for inspection via the serialscan path from the TDI input pin of DBM1 20 to the TDO output pin DBM222.

Similarly, IC2 12 can be made to output data on the data bus leads 16 tobe captured and shifted out for inspection by the first DBM 20. In thisoff-line test mode, control to capture data and operate the scan path isinput via the TCK and TMS test bus input pins.

When the circuit of FIG. 1 is on-line and functioning normally, thefirst and second DBMs 20 and 22 can continue to monitor the data andaddress buses 14 and 16 using the internal EQM circuitry describedbelow. During on-line monitoring the internal EQM of each DBM device 20and 22 outputs control signals to capture the data appearing on the ODIinputs of the respective DBMs. The internal EQM operates synchronous tothe control signal outputs from IC1 10 which are input to each DBM viathe clock or CK inputs. To know when to capture data, the EQM circuitrywithin each DBM 20 and 22 has comparator logic which can match the dataappearing on the ODI inputs against a known or certain expected datapattern or set of expected data patterns.

To expand the event qualifying capability, multiple DBMs (or otherdevices containing the EQM and the EQI and EQO pins) can be connectedtogether on an external combining network, such as AND gate 24, to allowthe qualification of a test monitor operation to be controlled by theevents detected over a range of DBM devices. When expanded qualificationis required, each DBM outputs a match condition signal on its EQO outputpin. The EQO outputs of multiple DBMs are input to an external combiningcircuit or AND gate 24 to produce a global event qualifier (EQI) inputsignal that is fed back into each DBM via their EQI input pins. When amatched signal is input on the EQI pin, the internal EQM can initiate atest monitor operation. The operation and protocol of the EventQualification Module are described in U.S. Pat. No. 5,001,713, entitled“Event Qualified Testing Architecture For Integrated Circuits,” and U.S.Pat. No. 5,103,450, entitled “Event Qualified Testing Protocols ForIntegrated Circuits,” both filed Feb. 8, 1989 by Whetsel, both of whichare incorporated by reference into this patent.

The test bus controller 25 can control the shifting of data through theDBMs (and other devices). The combined EQI signal is monitored by thetest bus controller 25 to determine when a known or certain conditionoccurs. In response to one or more known or certain conditions, the testbus controller can scan out the data stored in the DBM and otherdevices. Test protocols and conditions are discussed in connection withFIGS. 7 a-d.

Digital Bus Monitor Architecture

FIG. 2 illustrates a block diagram of the DBM of the present invention,referenced generally as 20. The DBM integrated circuit comprises a testcell register control circuit (TCR control) 26, test cell register (TCR)28, memory buffer 30 (memory), event qualification module (EQM) 32,bypass register (bypass) 34, command register (command) 36, and testport 38. The DBM 20 has the following inputs and outputs: feedback input(FBI), ODI, EQI, clock 1 (CK1), clock 2 (CK2), TDI, TMS, TCK, feedbackoutput (FBO), EQO, and TDO. FBI is input to the TCR 28. ODI is input tothe TCR 28 and the memory, EQI is input to the EQM 32. A firstmultiplexer (MX1) 40 receives inputs from CK1 and CK2, and outputs aCK1/2 signal to the EQM 32. TDI is input to the Command Register 36,Bypass 34, EQM 32, memory 30, TCR 28 and TCR Control 26. TMS and TCKsignals are input to the Test Port 38. The command register 36 isconnected to the test port 38, MX1 40, a second multiplexer (MX2) 42, abypass register 34, EQM 32, memory buffer 30, TCR 28 and TCR control 26via a command bus 44. The test port 38 is connected to a thirdmultiplexer (MX3) 46, the command register 36, bypass register 34, EQM32, memory 30, TCR 28, and TCR control 26 via a test bus 48. The serialoutputs of the data registers TCR control 26, TCR 28, memory 30, EQM 32and bypass 34 are connected to the inputs of MX2 42 which is controlledby the signal from the command bus 44. The output of MX2 42 is connectedto a first input to MX3 46. The other input to MX3 46 is received fromthe serial data output of the command register 36. The output of MX3 46is connected to the TDO output. The EQO output is provided by the EQM 32and the FBO output is received from TCR 28. TCR 28 receives controlsignals from TCR control 26 over control bus 50. EQM 32 outputs signalsto memory 30 and TCR 28 via EOM bus 52. EQM 32 receives signals from TCR28 via TCR bus 54.

The DBM Architecture has a four-wire test bus input (TCK, TMS, TDI andTDO) and parallel access of multiple internal scan paths which isconsistent with the proposed P1149.1 IEEE standard. When control isinput to the test port 38 via the TMS and TCK inputs, serial data isloaded and shifted in from the TDI input through either the commandregister 36 or one of the selectable data registers 26-34 from the TDIinput to the TDO output.

The test port 38 responds to the test bus protocol which is input viathe external TMS and TCK; inputs to load and then shift data througheither the command register 36 or one of the five selectable dataregisters 26-34. Control input from command register 36 via control bus44—is input to the test port 38. This control input enables the clockoutput to a selected data register 26-34 during a data register scanoperation. The selected data register clock output can also be used tocontrol the EQM 32 and/or TCR 28 during off line test monitoroperations. The data and command register clock outputs, along withother control signals, are output from the test port 38 via control bus48.

When selected, the command register receives control from the test port38 via control bus 48 to shift data from the TDI input to the TDOoutput. The instruction shifted into command register 36 is used toselect one of the data registers 26-34 to be connected to the TDO outputvia multiplexers MX2 42 and MX3 46. When selected, a data register canreceive control from the test port 38 via control bus 48 to shift datafrom the TDI input to the TDO output. In addition to selecting a dataregister for scan access, the instruction in the command register 36 canalso output the control required to execute a test monitor operation inthe DBM 20.

Bypass register 34 comprises a single scan cell. When selected viacommand bus 44 and control bus 48, bypass register 34 couples the TDIinput to the TDO output through a single shift register stage. Thebypass register 34 is used to provide an abbreviated scan path throughthe DBM.

The EQM 32 is a circuit which is used to control the operation of theTCR 28 and memory 30 during on line test monitoring operations. The EQM32 receives input from the external EQI input, a selectable clock outputsignal from MX1 40, a compare term (CTERM) signal output from TCR 28,command bus 44, and control bus 48. The EQM 32 outputs test controlsignals to TCR 28 and memory 30 via bus 52, and also outputs compareresults on the external EQO output signal. The EQM 32 is described ingreater detail in connection with FIG. 7.

The memory 30 is used to store incoming data during test monitoroperations. The memory 30 receives input from the external ODI input,the command bus 44, the EQM bus 52, and the control bus 48.

The TCR 28 is used to capture or compress incoming data during testmonitor operations. The TCR 28 receives input from the external FBIsignal, the external ODI input, the command bus 44, the EQM bus 52, andthe control bus 48. The TCR outputs an external FBO signal and a CTERMsignal to the EQM 32.

The TCR control register 26 is used to store configuration signals thatare used to mask off ODI inputs to the TCR 28, select the TCR'spolynomial tap configuration and adjust the TCR 28 for cascading withTCRs in other DBM devices. The TCR control register 26 receives inputfrom the command bus 44, and control bus 48. The TCR control register 26outputs control to the TCR 28 via bus 50.

MX1 40 is used to select one of the two external clock inputs to beapplied to the EQM 32. MX1 40 has one output, CK1/2 and two inputs,external CK1 input and external CK2 input. MX1 40 is controlled by thecommand register 36 via the command bus 44. It should be noted that morethan two external clocks can be input to the DEM 20, depending upon theavailable pins. The additional clocks allow the DBM to synchronize withmultiple timing sources.

MX2 42 is used to select one of the serial outputs from the fiveselectable data registers 26-34 to be input to MX3 46. MX2 42 selectsits output from the serial data outputs the bypass register 34, EQM 32,memory 30, TCR 28 and TCR control register 26, MX2 42 is controlled bythe command register 36 via the command bus MX3 46 is used to couple theserial data output from the command register 36 or the output of MX2 42to the external TDO output. MX3 is controlled by the test port 38 viathe test bus 48.

DBM Input/Output Descriptions

The DBM's TDI input and TDO output of FIG. 2 are wired such that aleading device's TDO output drives into the DBM's TDI input and afollowing device's TDI input is driven by the DBM's TDO output. The TCKand TMS input signals are connected in parallel to multiple DBMs orother devices with the same standard test bus interface. An example ofthis interconnect scheme is in FIG. 1.

The ODI inputs of the DBM in FIG. 2 are connected to a digital bus paththat is to be monitored (see FIG. 1). For purposes of illustration, itis assumed that the DBM 20 has an ODI input bus width of 16 bits, toallow simultaneous monitoring of 16 bit wide buses. However, DBMs couldbe designed with wider ODI input bus widths to allow monitoring of buseswider than 16 bits. By cascading 16 bit DBMs, it is possible to monitorbus widths of 32, 48, 64, etc. Also, the data compression capability ofthe internal TCR can be expanded in multiples of 16 bits by cascading aseries of DBMs together and making the appropriate feedback wiringconnections on the DBMs FBI and FBO signal pins.

The DBM's CK1 and CK2 input pins are connected to a system clock sourcewhich is synchronous to occurrence of valid data on the ODI inputs. Thetwo clock inputs, CK1 and CK2, allow selection of one of two possibleclock source inputs to be used during on line monitoring. The selectedclock source is routed through MX1 42 and is input to the EQM 32. Duringon line monitoring the EQM 32 operates synchronously with the selectedclock input to issue control to the TCR 28 and/or memory 30 to capturethe data appearing on the ODI inputs. If required, clock signals orother control signals in addition to the two clock signals illustratedcould be input to the DBM.

The DBMs' EQI inputs and EQO outputs are connected to an external logicand feedback network comprising an AND gate 24 as shown in FIG. 1. Theexternal feedback allows multiple DBMs and/or other devices that includethe event qualification architecture to operate together to qualify andon line test operation. Although an AND gate 24 is illustrated in FIG. 1for the combining circuit other logic circuits, such as an OR gate couldbe used for the external feedback network. For example, if the EQOsignals output a logic “1” in response to a match, an AND gate willdetect the occurrance of all ones. However, if EQO signals output alogic “0” in response to a match, an OR gate can be used to detect allzeros.

Test Port

In FIG. 3, the design of the test port is shown. The test port comprisesa control section 56 and a clock select section 58. The control sectionreceives external control from the TMS and TCK inputs and outputsinternal control on the load/shift (L/S) control output, an instructionclock output (IRCK) and data clock outputs (DRCK). In the preferredembodiment, the control section 56 conforms to the proposed IEEE testbus standard protocol to load and shift data through either the commandregister 36 or a selected data register 26-34. The clock select sectionreceives inputs from command bus 44 and DRCK, and outputs five clocks(DRCK1-DRCK5) onto the control bus 48.

In FIG. 3 a a load operation occurs on the command register when the L/Scontrol output is high and the IRCK clock is applied. After the loadoperation has occurred, the L/S control output is set low to shift outthe data loaded during each IRCK clock output. After the shift operationis complete, the IRCK clock output is set low, and the L/S outputreturns high.

In FIG. 3 b, a load operation occurs on a selected data register 26-34when the L/S output is high and the DRCK clock is applied. After theload operation has occurred, the L/S control output is set low to shiftout the data loaded during each DRCK clock output. After the shiftoperation is complete, the DRCK outputs are set Low and the L/S outputreturns high.

The instruction Loaded into the command register 36 selects which dataregister 26-34 will receive clock inputs during data register scanoperations. The control to select one of the data register clocks(DRCK1-5) is input to the select logic 58 via the command bus 44. Theselected data register clock will transition with the DRCK during dataregister scan operations.

Command Register

FIG. 4 illustrates the design of the command register 36. The commandregister 3 is used to store test instructions. The command register 36receives serial input on the TDI input and control input from the testport 38 via the L/S and IRCK signals on the control bus 48. The commandregister 38 outputs serial data on the TDO output signal. The commandregister comprises a series of scan cells, similar to the bypass scancell 60 shown in FIG. 5, interconnected together to form a shiftregister.

During instruction register scan operations, the L/S and IRCK outputsfrom the test port 38 are activated to load and shift data through thecommand register 36 from the TDI input to the TDO output. During theload operation, the L/S and IRCK inputs cause the command register cellsto load the data attached to one input of a 2:1 multiplexer. The dataloaded can be either a fixed binary pattern or variable status inputs.After the load operation, the L/S control input selects the other inputof the 2:1 multiplexer to link the scan cells together to shift datathrough the DBM from the TDI input, through the instruction register tothe TDO output, and then to the MX3 46, where the data is selectivelyoutput to the TDO signal.

Bypass Register

FIG. 5 illustrates the design of the bypass register 34. The bypassregister 34 comprises a single scan cell 60 and is used to shorten theDBM's data scan path to only a one bit length. The scan cell 60comprises a 2:1 multiplexer 62 and a D flip flop 64. When selected, thescan cell 60 receives L/S and DRCK1 control inputs from the test port 38via control bus 48 to load the data attached to one input of the 2:1multiplexer 62. After the load operation, the L/S control input selectsthe other input of the 2:1 multiplexer 62 to shift data through the DBM20 from the TDI input, through the bypass register 34 scan cell to theTDO output of the bypass register 34, and then to the TDO 106 output ofthe DBM 20, via MX2 42 and MX3 46.

TCR Control Register

FIG. 6 illustrates the design of the TCR control register 50. The TCRcontrol register 50 comprises a series of scan cells similar to thebypass scan cell 60 shown in FIG. 5. The TCR control register 50receives serial input on the TDI input and control input from the testport 38 via the L/S and DRCK5 signals on the control bus 48. The TCRcontrol register 50 outputs serial data on the TDO output signal. Whenselected, the scan cells in the TCR control register receive L/S andDRCK5 control signals to load the data attached to one input of a 2:1multiplexer. In the TCR control register 50 the multiplexer inputselected while L/S is high is attached to the scan cell's Q output sothat the cells remain in their present state during the load operation.After the load operation, the L/S control input selects the other inputof the 2:1 multiplexer to link the scan cells together to shift datathrough the DBM from the TDI input, through the TCR control register, tothe TDO output, and then to the TDO output of the DBM.

Event Qualification Module

FIG. 7 illustrates a block diagram of the EQM 32. The EOM receives sevensignals: TDI, L/S, DRCK2, CTERM (from TCR 28), EQI, EQENA (from thecommand register 36) and CK1/2. The EQM 32 has six outputs: EQO, EXPDAT0-15, CMPMSK 0-15, TCATE, TDO, and SYNCK. The SYNCK output is generatedthrough AND gate 66, having inputs of TGATE, EQENA, and CK1/2. Adetailed description of the EQM is provided in U.S. patent applicationSer. No. 308,272, referenced above.

The EQM 32 has a scan path which contains an EQM command register, anevent loop counter, a configuration bits register, start and stopexpected compare data registers, and start and stop mask compare dataregisters. When selected, the L/S and DRCK2 control inputs from the testport 38 via control bus 48 cause the EQM scan path to load and shiftdata from the TDI input, through the EQM 32 to the TDO of the EQM 32,and then to the TDO output of the DBM 20 via MX2 42 and MX3 46.

The EQM 32 receives condition input from the internal CTERM signal andexternal EQI signal. The EQM 32 can respond to a condition input on aselected one of these two condition inputs to execute an on line eventqualified test monitor operation. The EQM receives external clock inputfrom the CK1/2 output from MX1 40 in FIG. 2. The EQM 32 operatessynchronous to CK1/2 input during execution of an on-line eventqualified test monitor operation. The EQM 32 receives input from the EQMenable (EQENA) signal output from the command bus 44. When EQENA is sethigh, the EQM controller (internal to the EQM 32) is enabled to outputthe required control, in response to a condition input, to execute anevent qualified test monitor operation in the TCR and/or memory buffer.

When the EQENA is set high, the EQM is enabled to output expectedcompare data (EXPDAT) and mask compare data (CMPMSK) to the TCR 28. TheEXPDAT pattern is used to compare the input signals appearing on the ODIinputs against an expected input pattern. The CMPMSK pattern is used tomask off a compare operation on one or more of the ODI input signals, towhere it has no effect on the compare operation being performed. In thepreferred embodiment, the EQM 32 has storage to hold multiple sets ofEXPDAT and CMPMSK data patterns.

When a match is found between incoming data on the ODI inputs and theEXPDAT, the EQM 32 outputs a high logic level on the TGATE output. TheTGATE output is routed to the TCR 28 and memory 30 to enable a testmonitor operation. Also, when the TGATE output is high, the AND gate 66in FIG. 7 is enabled to pass the CK1/2 clock input to the SYNCK signal.The SYNCK signal is routed to the TCR 28 and memory 30 to Provideclocking for an on-line test monitor operation. In addition, the EQM 32outputs the occurrence of a match condition on the external EQO outputsignal to inform neighboring devices of the match. The EQO signal can beused to qualify a more global event qualified test operation using theexternal AND feedback network 24 shown in FIG. 1.

The EQM 32 can perform four types of event qualified testing protocols.Timing diagrams for each of the four types of protocols are shown inFIGS. 7 a, 7 b, 7 c and 7 d. A protocol 1 operation allows forperforming a single test monitor operation in response to a conditioninput. A protocol 2 operation allows for performing a test monitoroperation while the condition input is present. A protocol 3 operationallows for performing a test monitor operation over an interval of timebetween a start condition input and a stop condition input. A protocol 4operation allows for performing a test monitor operation which can bestarted with a first condition input, paused with a second conditioninput, resumed with a third condition input, and stopped with a fourthcondition input. All the protocols can be made to repeat a desirednumber of times as determined by the EQM's internal event loop counter.The operation of the EQM and its protocols are described in detail inU.S. Pat. No. 5,001,713 and U.S. Pat. No. 5,103,450, referenced above.

Memory Buffer

FIG. 8 illustrates a block diagram of the memory buffer 30. The memorybuffer 30 receives the L/S and DRCK3 via the control bus 48, a MODE1,ENA, CNTEN, DISEL, and LDSEL signals via the command bus 44, the SYNCKand TGATE signals from the EQM 32 via the EQM bus 52, the TDI signal andthe ODI signals (shown for a sixteen-bit implementation). The memorybuffer 30 outputs a TDO signal. A first multiplexer 68 has inputsreceived from a data register 70 and a RAM memory 72. The firstmultiplexer 68 is under the control of the LDSEL signal. The output ofthe first multiplexer 68 is connected with the data register 70. Asecond multiplexer 74 has one input connected to the data register 70and the other input connected to the ODI signal. The output of thesecond multiplexer is connected to the RAM 72 under control of the DISELsignal. The L/S signal is also connected to an AND gate 76 along with anENA signal. The output of the AND gate 76 is connected to a thirdmultiplexer 78 along with the TGATE signal. The DRCK3 signal and SYNCKsignal are input to a fourth multiplexer 80. Both the third multiplexer78 and fourth multiplexer 80 are controlled by the MODE1 signal. Theoutput of the third multiplexer 78 is connected to the write enable 10of the RAM 72. The output of the fourth multiplexer 80 is connected tothe read/write pin (WR) of the RAM 72. The output of the fourthmultiplexer 80 is also connected to the data register 70 and to anaddress/counter 81. The output of the address/counter 81 is connected tothe address pins of the RAM 72. The address/counter 81 is also connectedto the L/S signal, the CNTEN signal and to the data register 70. A TDOsignal is also output from the address/counter 81. The data register 70also is connected to the L/S signal, the output of the fourthmultiplexer 80, and to the TDI signal.

The memory buffer 30 comprises a static design random access memory(RAM) 72, a scan path, and required interface logic and multiplexers.The RAM memory 72 has a data input width equal to the number of ODIinput signals and sufficient depth for maximum data storage. The scanpath consists of a data register 70 and an address/counter 82. Whenselected, control inputs L/S and DRCK3 from the test port 38 via controlbus 48 causes the scan path to load and shift data from the TDI inputthrough the data register 70 and address/counter 81 to the TDO output ofthe memory 30, and then to the TDO output of the DBM 20 via the MX2 42and MX3 46. During scan operations, the MODE1 input from the bus 44 isset to allow the DRCK3 input to pass through the fourth multiplexer 80to clock the data register 70 and address/counter 81 sections of thescan path.

During a memory read instruction, the scan path load and shiftoperations are used to extract the contents of the RAM 72. During thisinstruction, the ENA input from the command bus 44 is low to disable RAMwrite operations, and the first multiplexer 68 is set by the load select(LDSEL) signal from command bus 44 to allow the data register 70 of thescan path to load the memory location addressed by the address/counter81. The address/counter section 81 remains in its present state duringthe scan load operation. After a load operation, the L/S input is setlow so that the scan can shift out the RAM data location and read andshift in the next RAM address to be read during the next scan pathload/shift operation. This scan path load/shift process is repeateduntil all the RAM memory locations have been loaded and shifted out.

During a memory write instruction, the scan path load and shiftoperations are used to Load data into the RAM 72. During thisinstruction, the first multiplexer 68 is set by the LDSEL input to allowthe data register 70 of the scan oath to remain in its present stateduring the load operation. Also, the second multiplexer 74 is set by thedata input select (DISEL) signal from command bus 44 to allow the RAMinputs to be driven by the data in the data register 70 of the scanpath. Also, the ENA input from the command bus 44 is set high to enablethe RAM 72 to accept data input during the scan path load operation.During the load operation, the RAM write enable (WE) input is high bythe ENA and L/S inputs being high, and the DRCK3 clock pulse causes theRAM 72 to accept the data input from the second multiplexer 74 into thelocation addressed by the address/counter 81. After the load operation,the scan path is shifted to load the next data and address pattern.Since the L/S input goes low during the shift operation, the WE input islow and the DRCK3 inputs that occur during shifting do not cause furtherRAM write operations. This load/shift process is repeated until the RAMmemory has been filled.

During off line data buffering operations, the CNTEN input from thecommand bus 44 is set high to enable the address/counter for count upoperations. The MODE1 input is set to allow the DRCK3 input to drive theRAM's WR input, data register 70, and address/counter 81 via the fourthmultiplexer 80. The LDSEL input is set so that the data register 70remains in its present state when DRCK3 clocks are applied. Also, theMODE1 allows the high logic level inputs on the ENA and L/S signals toactivate the RAM WE input through the third multiplexer 78. The DISELinput is set to allow the ODI signals to be input to the RAM 72 via thesecond multiplexer 74.

The address/counter 81 is set to zero prior to executing a databuffering operation. During off line data buffering, test port 38 is setup so that external control input via the TMS and TCK signals canactivate clock pulses on the DRCK3 output. When a high clock pulseoccurs on the DRCK3 signal, data on the ODI inputs are written into thecurrently addressed RAM location. When the clock pulse on DRCK3 returnslow, the address/counter 82 increments to the next RAM address location.This process of storing the data input on the ODI signals, followed byincrementing the address/counter 81, is repeated while DRCK3 clockinputs are enabled via the external test bus control signals.

During on line data buffering operations, the CNTEN input is set high toenable the address/counter 81 for count up operations. The MODE1 inputis set to allow the gated SYNCK input from the EQM 32 to drive the RAMWR input, data register 70 and address/counter 81. The LDSEL input isset so that the data register 70 remains in its present state when SYNCKclocks are applied. The SYNCK clock input is enabled while the TGATEinput from the EQM 32 is high and is gated off while the TGATE input islow. Also, the MODE1 input is set to allow the TGATE input from the EQM32 to activate to the RAM WE input via the third multiplexer 78, whileTGATE is high. The DISEL input is set to allow the ODI signals to beinput to the RAM via the second multiplexer 74.

The address/counter 81 is set to zero prior to executing a databuffering operation. On line data buffering is initiated when the EQM 32sets the TGATE signal high. While TGATE is high, the RAM WE is high andSYNCK clocks are enabled to clock the RAM 72 and address/counter 81.When a high clock pulse appears on the SYNCK clock, data on the ODIinputs are written into the currently addressed RAM location. When theSYNCK clock returns low, the address/counter 81 increments to the nextRAM address location. This process of storing the data input on the ODIsignals, followed by incrementing the address/counter 81 is repeated,while the TGATE input signal from the EQM 32 is set high.

Test Cell Register (TCR)

FIG. 9 illustrates a block diagram of the TCR 28. The TCR 28 includes aregister 82 comprised of a series of test cells interconnected to form atest register. The test register 82 is described in detail in U.S. Pat.No. 5,495,487 to Whetsel, entitled “Testing Buffer/Register,” filed Sep.7, 1988, which is incorporated by reference into this patent. Each testcell in the test register 82 is connected to a respective bit (0-15) ofthe EXPDAT, CMPMSK, DATMSK and ODI signals. The test cell register 82also receives a PSAENA signal from the command register 36 via thecommand bus 44 and the output of first, second and third multiplexers84, 86, and 88. Multiplexer 84 receives the TDI input and an FBO signal.The FBO signal received by the multiplexer 84 is the output of anexclusive or network 90, which is part of the TCR 28. The multiplexer 84is controlled by the FBSEL signal from the TCR control register 26.Multiplexer 86 has the L/S and TGATE signals as inputs and is controlledby the MODE2 signal from the command register 36 via the command bus 44.Multiplexer 88 receives the DRCK4 and SYNCK signals and is alsocontrolled by the MODE2 signal. The EXOR gate network 90 receivesTDO0-15 outputs from the test cell register 82, TAP 0-15 signals, andthe output of an AND gate 92. The AND gate 92 receives an FBIENA signalfrom the TCR control register 26 and the FBI signal. The test cellregister 82 outputs CMPOUT signals for each test cell in the register;the signals are input to an AND gate 94 which outputs the CTERM signal.The test cell register 82 also outputs a TDO signal. The number of testcells in the test register is equal to the number of ODI input signals.

The test register 82 receives input from the L/S and DRCK4 test portsignals to load and shift data through the test register 82 from the TDIinput to the TDO output of FIG. 9. During scan operations, the feedbackselect (FBSEL) signal from the TCR control register 26 is set to allowthe TDI signal to be input to the test register 82 via multiplexer 84.Also, during scan operations, the MODE2 input from the command register36 is set to allow the L/S and DRCK4 inputs to be passed throughmultiplexers 86 and 88 for input to the test register 82 via the SEL andCK inputs, respectively.

An example test cell design used to construct the test register is shownin FIG. 9 a. The test cell comprises a maskable PSA logic 96, maskablecomparator logic 98, and a scan cell 100 comprising a 2:1 multiplexer102 and a D flip flop 104. The maskable PSA logic 96 comprises NANDgates 106 and 108 and EXOR gate 110. NAND gate 06 receives inputs froman ODI input signal and a DATMSK input signal from the TCR controlregister 26, NAND gate 108 receives a PSAENA input signal from commandregister 36, and a TDI input signal from a leading scan cell's TDOoutput. The EXOR gate 110 receives the outputs of NAND gates 106 and 108and outputs a PSA signal to the 2:1 multiplexer 102.

The maskable comparator logic 98. comprises EXOR gate 112 and a NANDgate 114. EXOR gate 112 receives inputs from an ODI input signal and anEXPDAT input signals from the EQM 32. NAND gate 114 receives a CMPMSKsignal and the output of EXOR gate 112 and outputs a CMPOUT signal toindicate a match at that cell. The scan cell's 2:1 multiplexer 102receives the PSA signal and a TDI signal input to the test cell. Themultiplexer 102 outputs a signal to the D input of the D flip flop 104under control of the SEL signal from multiplexer 86 (FIG. 9). The D flipflop 104 receives a data input from the 2:1 multiplexer 102 and a clock(CK) input from the multiplexer 88 (FIG. 9). The operation of each ofthese logic sections is described in a truth table in TABLE I.

TABLE I Scan Cell Truth Table Sel Clk Operation 0 / Shift (TDI to TDO) 1/ Load (PSA to TDO) where “/” is a rising clock edge.

Referring to FIG. 9, the EXOR gate network 90 comprises an arrangementof AND gates 116 and EXOR gates 118 as shown in the exampleimplementation of FIG. 9 b. The EXOR gate network 90 receives input fromthe TDO outputs of the test cells in the test register 82, polynomialtap (TAP) select inputs from the TCR control register 26, and theexternal feedback input (FBI) (see FIG. 2). The EXOR gate network 90outputs a feedback output (FBO) which is input to the first test cell ofthe test register 82 via multiplexer 84, and it is also output from theDBM 20 on the FBO signal. The TAP 1-15 inputs to the EXOR network 90allow programmable selection of the TDO 0-15 outputs from the testregister 82 that are to be included in the calculation of the FBO signalused during PSA operations. The external FBI input signal can beincluded in the calculation of FBO if the FBIENA input from the TCRcontrol register 26 is set high; otherwise, data input on FBI is ignoredby the EXOR network 90.

Referring again to FIG. 9, the CMPOUTO-15 outputs From the test cells inthe test register are ANDed together and output from the TCR 28 via theCTERM signal. The CTERM signal is input to the EQM 32 as a conditioninput. The CMPOUTO-15 outputs reflect the result of a compare operationbetween ODIO-15 and EXPDATO-15 signal buses input to the TCR 28. TheCMPMSKO-15 inputs can be used to mask off one or more of these compareoperations. When a compare logic section is masked off, its CMPOUToutput is set to a high logic level.

The PSAENA input signal from the command register 36 to the testregister 82 is used to select whether the test cells perform a PSA orsample operation. If PSAENA is set high, all test cells are selected forPSA operation. If PSAENA is low, all test cells are set for sampleoperation. The DATMSKO-15 signals from the TCR control register 26 areused to mask off one or more of the ODIO-15 signal inputs during eithera PSA or sample operation. If a DATMSK input is low, the ODI inputassociated with it is masked off, and the test cell loads a low logiclevel during PSA or sample operations. If a DATMSK input is high, thelogic level of the ODI input it is associated with is loaded into thetest cell during a PSA or sample operation.

During off line PSA or sample instructions, the MODE2 input signal willbe set to allow the L/S and DRCK4 signals from the test port 38 to beinput to the test register 82 via multiplexers 86 and 88, respectively.During this test mode the L/S input signal will be set high, and theDRCK4 clock will be enabled by the external input to the test port 38.If a PSA operation is being performed, the PSAENA input to the testregister is set high. If a sample operation is performed, the PSAENAinput is set low. The data appearing on the ODIO-15 inputs is clockedinto the test register cells during each high clock pulse on the DRCK4input. After the PSA or sample operation is complete, the data orsignature collected is shifted out for inspection via a TCR readinstruction.

During on-line PSA or sample instructions, the mode 2 input signal willbe set to allow the TGATE and SYNCK signals from the EQM 32 to be inputto the test register via multiplexers 86 and 88. During this test mode,the TGATE signal will be set high to enable the SYNCK and to select thePSA/sample mode of operation in the test cells of the test register. TheTGATE signal is set high in response to an input condition according tothe type of protocol selected as shown in FIG. 7 a, 7 b, 7 c, or 7 d. Ifa PSA operation is being performed, the PSAENA input to the testregister will be set high. If a sample operation is performed, thePSAENA is set low. The data appearing on the ODIO-15 inputs is clockedinto the test register cells during each high clock pulse on the SYNCKinput. After the PSA or sample operation is complete, the data orsignature collected is shifted out for inspection via a TCR readinstruction.

It should be noted that data can be stored sequentially in the memorybuffer 30 while it is being compressed in the TCR 28.

The TCR 28 and memory buffer 30 may be controlled by both the externaltest bus controller 25 and the EQM 32. Since the external test buscontroller 25 may be controlling a large number of DBMs and otherdevices, the internal EQM 32 provides the detailed control for each DBM.

Advantages of Maskable PSA Inputs

In FIG. 9 c, the TCR of a DBM device is shown collecting a signature onmultiple parallel ODI inputs. In this mode, the DATMSK 0-15 inputs tothe TCR 28 from the TCR control register 26 are set to allow all ODIinput signals to be involved in producing a signature value. After thesignature is collected, it is shifted out and compared to what thesignature should be equal to. If the signature collected does not matchthe correct value, a fault has occurred on one or more of the ODIinputs. It is difficult, if not impossible, to determine which ODI inputor groups of inputs caused the signature to fail.

Since the DBM can mask off one or more of the ODI inputs, it is possibleto repeat the test operation with only one ODI input enabled at a time.This capability can be used to collect a signature of one single ODIinput at a time. Using this technique, it is possible to diagnose whichODI input or inputs caused the parallel signature to fail.

In FIG. 9 d, an example is shown wherein all the ODI input signals aremasked off except for one, ODIO. By repeating the same test that failedwith one ODI input enabled, a signature can be produced which reflectsthe data input only by the single enabled ODI input. After this singleinput signature is taken, it can be shifted out and compared to anexpected signature for that ODI input. If it matches the expectedsignature, the ODIO input is good and did not cause the parallel inputsignature to fail. If it does not match, it, along with otherpotentially bad ODI inputs, caused the parallel input signature to fail.

This process is repeated by enabling another single ODI input signalwhile masking off all others and repeating the test again to collect asignature from the enabled ODI input signal and comparing it to what itshould be equal to. By following this procedure for all ODI inputs, itis possible to determine which ODI input or inputs caused the parallelsignature to fail. This technique is useful in the diagnosis of failedparallel signature analysis operations. Without the capability toselectively enable one ODI input signal at a time, it would not bepossible to employ this simple but thorough diagnostic test technique.It should be noted that this diagnostic test could also be performed byisolating groups of ODI inputs.

Cascading DBMs to Expand PSA Signature Width

Expanding the width of the signature being taken can be performedthrough a series of coupled DBM integrated circuits as shown in FIG. 10.The DBMs in FIG. 10 are coupled in series from a most significant DBM(MSDBM) 120, through one of more middle DBMs (MIDBM) 122, to a leastsignificant DBM (LSDBM). The TDO of each preceding DBM is connected tothe TDI of the next DBM; the FBI of each preceding DBM is connected tothe FBO of the next DBM.

For the MSDBM 120 in FIG. 10, the FBIENA and FBSEL inputs to the TCR 28must both be set to a logic high level in cascaded applications. Withthis setting of FBIENA and FBSEL, the external FBI input to the MSDBM120 is input to the MSDBMs EXOR network 90 via the AND gate 92 of FIG. 9(GFBI), and the FBO output from the MSDBM's EXOR network 90 is input tothe test register 82 via multiplexer 84 of FIG. 9.

For the MIDBM 122 in FIG. 10, the FBIENA and FBSEL inputs to the TCR 28must be set to a logic high and low level, respectively, in cascadedapplications. With this setting of FBIENA and FBSEL, the external FBIinput to the MIDBM 320 is input to the MIDBM's EXOR network 90 via theAND gate 92 of FIG. 9 (GFBI), and the TDO output from the MSDBM is inputto the test register 82 of the MIDBM 122 via the TDI input to themultiplexer 84 of FIG. 9. The FBO output from the MIDBM 122 is input tothe FBI of the MSDBM 120.

For the LSDBM 124 in FIG. 10, the FBIENA and FBSEL inputs to the TCR 28must both be set to a logic low level in cascaded applications. Withthis setting of FBIENA and FBSEL, the external FBI input to the LSDBM124 is gated off and a low logic level is input to the LSDBM's EXORnetwork 90 via the AND gate in FIG. 9 (GFBI), and the TDO output fromthe MIDBM 122 is input to the test register 82 of the LSDBM 124 via theTDI input to multiplexer 84 of FIG. 9. The FBO output from the LSDBM 124is input to the FBI of the MIDBM 122.

During non-cascaded PSA configurations, the FBIENA and FBSEL input tothe TCR's of the MSDBM 120, MIDBM 122, and LSDBM 124 will be set to alow and high logic level, respectively. With this setting of FBIENA andFBSEL, the external FBI input to the TCR 88 is gated off, and a lowlogic level is input to the EXOR network 90 via the AND gate 92 in FIG.9 (GFBI), and the internal FBO from the EXOR network is input to thetest register 82 via multiplexer 84 in FIG. 9.

DBM Instructions

When an instruction is shifted into the command register 36, theappropriate control is output via the control bus 48 to execute a testor scan operation in one or more of the selectable data register scanpaths; bypass register 34, EQM 32, memory buffer 30, TCR 28, or TCRcontrol register 26.

During the off line data sample instruction, the TCR 28 receives controlfrom the external test bus via test port 38 and control bus 48 tocapture the data appearing on the ODI inputs. After the data iscaptured, it can be shifted out of the TCR for inspection via a TCR readinstruction.

During the on line data sample instruction, the TCR 28 receives controlfrom the internal ECM 32 via EQM control bus 52 to capture the dataappearing on the ODI inputs. After the data is captured, it can beshifted out of the TCR 28 for inspection via a TCR read instruction.

During the off line parallel signature analysis (PSA) test instruction,the TCR 28 receives control from the external test bus via test port 38and control bus 48 to compress the data appearing on the ODI inputs intoa 16 bit signature. After the data is compressed, the resultingsignature can be shifted out of the TCR 28 for inspect on via a TCR readinstruction.

During the on-line PSA test instruction, the TCR 28 receives controlfrom the internal EQM 32 via EQM control bus S2 to compress the dataappearing on the ODI inputs Into a 16 bit signature. After the data iscompressed, the resulting signature can be shifted out of the TCR forinspection via a TCR read instruction.

During the off-line data buffering instruction, the memory buffer 30receives control from the external test bus via test port 38 and controlbus 48 to store the data appearing on the ODI inputs into the internalRAM memory buffer 72. After the data has been stored, the memorycontents can be shifted out for inspection via a memory readinstruction.

During the on line data buffering instruction, the memory buffer 30receives control from the internal EQM 32 via EQM control bus 52 tostore the data appearing on the ODI inputs into the internal RAM memorybuffer 72. After the data has been stored, the memory contents can beshifted out For inspection via a memory read instruction.

During the off line PSA and buffer instruction, the TCR 28 and thememory buffer 30 receive control from the external test bus via the testport 38 and control bus 48 to compress and store, respectively, the dataappearing on the ODI inputs. After the data has been compressed into theTCR 28 and stored into the RAM memory 72, it can be shifted out forinspection by executing a TCR read instruction to access the signaturein the TCR 28, followed by executing a memory read instruction to accessthe memory contents.

During the on line PSA and buffer instruction, the TCR 28 and the memorybuffer 30 receive control from the internal EQM 32 via EQM control bus52 to compress and store, respectively, the data appearing on the ODIinputs. After the data has been compressed into the TCR and stored intothe RAM memory 72, it can be shifted out for inspection by executing aTCR read instruction to access the signature in the TCR 28, followed byexecuting a memory read instruction to access the memory contents.

During the TCR read instruction, the TCR 28 receives control from theexternal test bus via the test port 38 and control bus 48 to shift datafrom the TDI input, through the TCR 28, and out of the DBM 20 via theTDO output pin. This instruction is used to access the data captured orcompressed in the TCR during a data sample or PSA instruction.

During the TCR control scan instruction, the TCR control register 26receives control from the external test bus via the test port 38 andcontrol bus 48 to shift data from the TDI input, through the TCR controlregister 26, and out of the DBM 20 via the TDO output pin. Thisinstruction is used to load control bit signals required to set up theTCR's input data mask logic and polynomial feedback tap connections fora particular data sample or PSA test operation.

During the bypass scan instruction, the bypass register 34 receivescontrol from the external test bus via the test port 38 and control bus48 to shift data from the TDI input, through the bypass scan cell 34,and out of the DBM 20 via the TDO output pin. This instruction is usedto shorten the scan path through the DBM to only a single scan cell orflip flop.

During the memory read instruction, the memory buffer 10 receivescontrol from the external test bus via the test port 38 and control bus48 to load and shift out the currently addressed memory location via theTDI input and TDO output pins. During the shift out operation the nextmemory address to be read is shifted into the memory buffer 30.

During the memory write instruction, the memory buffer 30 receivescontrol from the external test bus via the test port 38 and control bus48 to load the contents in the data section of memory scan path into thememory location addressed by the address section of the memory scanpath. This instruction is used to initialize the memory buffer to aknown state for test monitoring and self testing purposes.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade herein without departing from the spirit and scope of the inventionas defined by the appended claims.

1. An integrated circuit comprising: A. a substrate of semiconductormaterial; B. operational circuitry formed on the substrate andperforming desired operations; C. scan circuitry formed on the substrateand connected to at least some of the operational circuitry, the scancircuitry including: i. a scan data input terminal, a scan data outputterminal, a scan clock terminal, and a scan mode select terminal; ii. ascan access port connected to the scan clock terminal and the scan modeselect terminal, the scan access port having control outputs; iii. aninstruction register and a first multiplexer connected in series betweenthe scan data input terminal and the scan data output terminal, theinstruction register having control outputs, the instruction registerand the first multiplexer having control inputs connected to the scanaccess port; iv. a bypass register and a second multiplexer connected inseries between the scan data input terminal and the first multiplexer,the bypass register having control inputs connected to the scan accessport, and the second multiplexer having control inputs connected to theinstruction register; v. a scan data register connected in seriesbetween the scan data input terminal and the second multiplexer, thescan data register having control inputs connected to the scan accessport; and vi. a data memory having one of a data input and data outputcoupled to the scan data register and having control inputs coupled tocontrol outputs of the scan access port.
 2. The integrated circuit ofclaim 1 in which the data memory inputs data from the scan data registerin response to control input from the scan access port.
 3. Theintegrated circuit of claim 1 in which the data memory outputs data tothe scan data register in response to control input from the scan accessport.
 4. The integrated circuit of claim 1 in which the data memoryfurther includes an address input coupled to the scan data register foraddressing locations within the data memory.
 5. The integrated circuitof claim 1 in which the data input selectively inputs data from a datasource separate from the scan data register.